Rapid miniaturization of portable electronic devices has resulted in an increased demand for a battery used as a power supply in the portable electronic devices to be lighter and thinner. Furthermore, development of a secondary battery having a small size, being lightweight, being charged and discharged for a long period of time, and having excellent high rate characteristics has been demanded.
A lithium secondary battery developed in the early 1990s has been spotlighted due to advantages such as, a high operation voltage and significantly high energy density as compared to a battery using an aqueous electrolyte such as, a Ni-MH battery, a Ni—Cd battery, a lead sulfate battery, and the like. However, in the lithium secondary battery, there can be safety issues such as, ignition and explosion of a non-aqueous electrolyte. As a capacity density of the battery is increased, this problem becomes more severe.
In a non-aqueous electrolyte secondary battery an issue may arise such as, safety deterioration of the battery which occurs at a time of continuous charge. One of the causes is heat generation due to collapse of a cathode structure. That is, a cathode active material of a non-aqueous electrolyte battery includes lithium, a lithium-containing metal oxide capable of intercalating and releasing lithium ions, and/or the like. As a large amount of lithium is detached at the time of over-charge, a structure of the cathode active material is changed to a thermally unstable structure. In this over-charge state, when a battery temperature reaches a critical temperature due to external physical impact, for example, exposure to a high temperature, oxygen is released from the cathode active material and the released oxygen generates an exothermic decomposition reaction with an electrolyte solvent. Particularly, since combustion of the electrolyte is further accelerated by oxygen released from the cathode, the battery may be ignited or ruptured due to thermal runaway caused by a series of exothermic reactions as described above.
To suppress the above-mentioned ignition or rupture due to an increase in temperature in the battery, a method of adding an aromatic compound to the electrolyte as a redox shuttle additive has been used. For example, a non-aqueous lithium ion battery, which is capable of preventing over-charge current and a thermal runaway phenomenon caused by the over-charge current by using an aromatic compound such as biphenyl, has been disclosed in Japanese Patent No. 2002-260725, which is herein incorporated by reference in its entirety. In addition, a method of improving safety of a battery by adding a small amount of an aromatic compound such as biphenyl, 3-chlorothiophene, or the like, to increase an internal resistance by electrochemical neutralization in an abnormal over-voltage state has been disclosed in U.S. Pat. No. 5,879,834, which is herein incorporated by reference in its entirety.
However, when using an additive such as biphenyl, or the like, there are issues in that when a relatively high voltage is locally generated during a general operation voltage, the additive is gradually decomposed during a charge and discharge process. When the battery is discharged at a high temperature for a long period of time, an amount of biphenyl, or the like, may gradually decrease. Thus, safety may not be secured, for example, after 300 charge and discharge cycles. In addition, an issue with storage characteristics may arise.
Therefore, research for improving safety at high and low temperatures while still having a high capacity retention rate has been demanded.